Select File › Import › Extract Images From PDF…
A dialog pops up to pick a file. Choose a .pdf file, and all its images will be opened as ImageJ images, in their original resolution.
If the PDF file is on the web, use a macro or script to call the plugin with the URL as argument:
For example, open the Jython Interpreter from Plugins / Scripting / Jython Interpreter (See Scripting Help), and paste the above with a desired .pdf URL argument.